1. Field of the Invention
The present invention relates to a small-sized motor having a polygonal outer shape for use in, for example, an air conditioner actuator or a motor-driven retractable mirror, and more particularly to a technique for mounting magnets for use as stator poles to the inner surface of a motor casing.
2. Description of the Related Art
As shown in FIG. 10A, an ordinary small-sized motor is such that a motor casing assumes a circular (round) tubular shape. When such a motor having a round outer shape (hereinafter, may be called a round motor) is to be mounted in an apparatus or on a wiring board, a whirl-stop must be employed for preventing rotation of the motor. When a round motor is mounted in an apparatus, the round shape tends to be accompanied by wasted space within the apparatus. In order to avoid rotation of a motor in relation to a mounting surface or to improve space efficiency, there is known impartment of a square or higher polygonal outer shape to a motor.
FIG. 7 shows the cross-sectional configuration of a conventional regular-polygonal motor (refer to Japanese Patent Application Laid-Open (kokai) No. 2005-20914). The illustrated regular-polygonal (hexagonal) motor is configured such that a tubular portion of a motor casing, which serves as a yoke, is formed into a regular-polygonal tubular shape and such that magnets serving as field poles (six poles) are attached to the respective inner surfaces of side portions of the motor casing, the side portions collectively forming a regular-polygonal tubular shape. In order to lower vibration of the side portions of the motor casing, the magnets are fixed to the respective side portions at the circumferential centers. A rotor having eight rotor poles is disposed radially inward of the magnets and is supported rotatably. Generally, individual magnets are shaped such that the radial clearance between the magnet and the outer circumferential surface of the rotor pole gradually increases from the circumferential center of the magnet toward the circumferentially opposite ends of the magnet. By virtue of this feature, the intensity of magnetic field is gradually weakened toward the opposite ends of the individual magnets, thereby avoiding an abrupt change in the intensity of magnetic field, which would otherwise result from rotation of the rotor. Thus, cogging torque can be lowered.
In order to lower cogging torque, the magnets of FIG. 7 are formed such that the distance between the magnet and the rotor is increased from the circumferential center of the magnet toward the circumferentially opposite ends of the magnet (along the circumference line shown by the alternate-long-and-short-dash line in FIG. 7). Even so, circumferentially opposite end portions of the magnet are still thicker than a circumferentially central portion of the magnet. Also, increasing the distance apparently causes a drop in motor performance. Furthermore, wasted spaces are formed between the magnets. Thus, as viewed on the cross section, disposing the magnets in the respective circumferentially central regions of the sides of the yoke raises the following problem: the magnets can be neither reduced in size nor arranged efficiently; consequently, the motor thickness (distance between diametrically opposed sides of the polygonal yoke) is increased, resulting in a failure to reduce the size of the motor.
In order to cope with the above problem, there is known a configuration in which, as viewed on the cross section of the yoke, the magnets are disposed at respective corners defined by adjacent sides of a polygonal yoke. FIGS. 8 and 9 are sectional views showing conventional motors in which the magnets are disposed at the respective corners defined by the adjacent sides (refer to Japanese Patent Application Laid-Open (kokai) No. H7-59322 and Japanese Utility Model Application Laid-Open (kokai) No. S64-12455). In FIG. 8, the motor casing has a square cross section, and a 4-pole field magnet magnetized with alternating north and south is disposed in the motor casing. This field magnet is magnetized such that the centers of magnetic poles are located at the respective corners defined by the adjacent sides of the yoke. A rotor having three rotor poles (arranged eccentrically) is rotatably supported within the field magnet.
In FIG. 9, four magnets for forming a 4-pole magnetic field are embedded at respective corner portions of the square yoke having beveled corners. A rotor is disposed radially inward of the magnets and is rotatably supported.
In order to improve productivity of small-sized motors, desirably, a field magnet is manufactured separately from a yoke and is then assembled with the yoke. However, in the case of the motor configuration shown in FIG. 8, making the field magnet fit corner portions of the yoke is not easy. Usually, such assembly employs a press-fit technique. However, in order to enable press-fit of the field magnet, a clearance must be provided between the field magnet and the yoke at the apexes of magnetic poles of the field magnet. Since the intensity of magnetic field must be enhanced particularly at the apexes of magnetic poles of the field magnet, the presence of such clearances causes a drop in motor performance. The shape of the field magnet is disadvantageous in that corner portions thereof fail to effectively function in relation to the rotor poles and that the weight and volume of the field magnet are increased, thereby causing a direct increase in cost.
The method of fixing the magnets through embedment in the yoke as shown in FIG. 9 involves the following problems: corner portions of the yoke are increased in thickness, so that the required amount of material is increased; and the complicated shape of the yoke causes an increase in the cost of machining a mold for the yoke. Also, insertion of the magnets may cause cracking of the yoke. Furthermore, a bonding process using an adhesive is required, and time for drying the adhesive is required as well.